The invention relates to a three-phase electrical machine, in particular a d.c. motor, comprising a first machine section which comprises an annular permanent-magnet body having a plurality of adjacent pole pairs, each comprising a north pole and a south pole, a second machine section which comprises a soft-magnetic body and at least one coil, in which body slots are formed to define teeth, which each have a tooth surface for cooperation with said poles via an air gap, the tooth surfaces having equal tangential dimensions, and a machine shaft about which one machine section is rotatable relative to the other machine section.
Such a machine in the form of a d.c. motor is known from U.S. Pat. No. 4,626,727 (herewith incorporated by reference), An embodiment of the prior-art motor comprises a stator formed by a star-shaped lamination assembly having six radial limbs which terminate in stator-tooth surfaces disposed on an imaginary cylindrical surface. The limbs carry coils. The rotor comprises a shaft carrying a disc-shaped body which changes into a cylindrical portion surrounding the stator and provided with an annular or cylindrical magnet on its inner side. The magnet, which is radially magnetised, has eight poles and cooperates with the stator tooth surfaces. Another embodiment of the prior-art motor comprises a stator having a star-shaped lamination assembly with radial limbs having axial end faces, which are used as stator tooth surfaces. The rotor in this embodiment is provided with a disc-shaped rotor support carrying an annular rotor magnet. The rotor magnet is axially magnetised and cooperates with the stator-tooth surfaces via an axial air gap. In this embodiment and in said other embodiment the ratio between the number of poles and the number of tooth surfaces is 4:3. In the known d.c. motor the pole pairs, which each comprise a north pole and a south pole, each extend tangentially over 360 electrical degrees. In this respect it is to be noted, that it is alternatively possible to provide a small gap between the successive pole pairs, so that each pole extends over less than 180 electrical degrees in a tangential direction. Indeed, German Offenlegungsschrift 31.22.049 (herewith incorporated by reference) also describes such a proposal.
A drawback of the known three-phase d.c. machine is the occurrence of detent torques. A detent torque arises because the overall magnetic energy varies when the pattern of the slots between the tooth surfaces and the pattern of north and south poles move relative to one another. As is known, the detent torque frequencies, expressed in the number of periods per revolution, correspond to the lowest common multiple of the poles and the teeth and to multiples thereof if the rotor and the stator of the d.c. machine are perfectly symmetrical. The frequency corresponding to the lowest common multiple is called the fundamental frequency; the frequencies corresponding to multiples thereof are referred to as harmonics. For uses where stringent requirements are imposed on a uniform operation of the rotor, for example in audio and video equipment, the occurrence of detent torques is undesirable. The fundamental frequency has the greatest influence on the occurrence of speed variations and is therefore most annoying.
A known method of eliminating detent torques is to make the pole separations between the poles inclined (skewing). A drawback of this step is that a certain loss of magnetic flux occurs and hence a loss of efficiency. Moreover, such pole separations may give rise to forces in undesired directions in specific motor constructions, which may produce noise. It is also known to provide narrow slots in conjunction with said skewing of the pole separations in order to reduce the effect the detent torque. However, narrow slots constitute a substantial drawback in the production of the motor, in particular in mounting the coils.
It is an object of the invention to modify the electrical machine defined in the opening paragraph in such a way that the fundamental frequency of the detent torque is wholly or almost wholly eliminated without the necessity of skewing the pole separations in the permanent-magnet body.